That green wax is called ambergris (term borrowed from a similar substance believed to originate from sick whales, and usually written as "ambergrease" in U.S. Army documents). It is in fact a chemical reaction that apparently is residual animal fat, tanning chemicals and preservative oils in the leather that the brass or bronze parts cause to combine with ambient moisture to create ambergrease.
I'm going to describe a solution that will probably have many people howling in protest because they think it will damage the leather and I have to admit I was apprehensive about trying this method. I have used this method on many different original equipment items (mostly 19th Century equipment) with success. I start by using dental tools and toothpicks to remove as much of the ambergris as possible. I then immerse the part in "Goof Off" for a day or two and the ambergrease is completely dissolved. The leather is left completely intact...that is no drying out or stain to mar the appearance. One dictionary I consulted stated that ambergrease is soluble in alcohol. I then treat the leather with a preservative and store the piece in an air tight container such as a "Ziploc" bag and purge as much of the air as I can.
I have at times worked a little of the Goof-off onto the affected area while using the toothpicks, but this doesn't remove the ambergrease as satisfactorily as immersion.
I have also used Goof Off to remove grease pencil, ink, and permanent marker applications from canvas and web equipment items, but with partial success as I do not immerse the item, but merely apply by external application.